The present invention relates generally to optical modulators and more specifically to an intensity modulator where phase dependant addition of signals results in amplitude changes.
To minimize distortion, amplitude (or intensity) modulated links require linear components such as optical modulators. Such analog links are useful in commercial applications for distributing cable TV signals and in military settings for remoting antennas/radar/etc.
Examples of current optical modulator technology are described in the following references, the disclosures of which are incorporated herein by reference:                U.S. Pat. No. 5,889,906 entitled Signal router with coupling of multiple waveguide modes for provicing a shaped multi-channel radiation pattern issued to Jerry Chen;        M. Nazarathy, J. Berger, A. J. Ley, I. M. Levi and Y. Kagan, J. Lightwave Technol. 11 Jan. 1993, pp. 82–105.        L. M. Johnson and H. V. Roussell, Opt. Lett. 13, October 1988, pp. 928–30.        H. Skeie and R. V. Johnson, Proc. SPIE: Integrated Optical Circuits 1583, Boston, Mass.: 3–4 Sep. 1991, pp. 153–64.        J. E. Zucker, M. Wegener, K. L. Jones, T. Y. Chang, N. Sauer, and D. S. Chemia, Appl. Phys. Lett. 56, 14 May 1990, pp. 151–3.        A. I. Plakhotnik, Opt. Spectrosc. (USSR) 68, March 1990, pp. 394–7.        U.S. Pat. No. 5,625,729, Apr. 29, 1997, Optoelectronic device for coupling between an external optical wave and a local optical wave for optical modulators and detectors, Brown, Thomas G.,        U.S. Pat. No. 5,355,422, Oct. 11, 1994, Broadband optical modulator, Sullivan, Charles T.,        Bown and Sullivan describe electro refractive optical modulators useable in the present invention.        
The above-cited references describe various modulators that use index changes to effect amplitude changes. A common example is the Mach Zehnder interferometer. Alternatively, one can use directional couples or Fabry Perot interferometers. Generally, the index changes depend linearly on the applied voltage. And all these modulators are inherently nonlinear giving rise to unwanted frequency harmonics. Researchers have sought to linearize them by predistorting the electrical input, and combining two or more in parallel and series. They have eliminated the 2nd and/or 3rd harmonic distortion products. Recently, interferometers with quadratic electro optic coefficients have been demonstrated in semiconductors with the quantum confined stark effect. To our knowledge, these electro refractive (a specific subset of quadratic electro-optic effect) devices have not been linearized yet, although Platonik suggested the suppression of second order harmonics in ceramics and lead magnoniobates, which also have quadratic electro optic effect. Here, we suggest, for the first time, a bias that suppresses third order harmonic distortion term. For sub octave signals, all the even harmonics can be filtered so the dominant nonlinearity is now the fifth harmonic.